It is generally known that measured data over an angular range of at least 180° are required in order to generate computed tomography displays from rotational scans of an examination object. However, as a rule measured data from a full circulation, that is to say a circulation above 360°, are used in order to reconstruct a CT slice image, or to reconstruct volume data. Since congruent beams with a complementary position of the X-ray source supply the same information, what is involved here is redundant data records that are certainly not mandatory in mathematical terms, but substantially improve the artifact behavior of such reconstructions. To this end, beams from one direction are interleaved with respectively complementary beams, that is to say beams in the opposite direction and, possibly an offset of the distance from the center of rotation.
It is possible in principle to use such radiation data in fan geometry directly and without rebinning for reconstruction. However, as a rule a starting point is the calculation in parallel geometry, which can, if appropriate, be produced by a so-called rebinning from data in fan geometry. Since such data do not satisfy the requirements of an equidistant distribution that is desired for an optimum reconstruction, the available projection data, which are present at different distances, are reinterpolated by a so-called radial interpolation on to an equidistant grid. Here, all the projection data are used with the same weight in accordance with their distance from the target point.
If dose rate data of the scanning X-ray tube are now modulated as a function of the scanning angle in order to reduce the surface dose of specific body regions, for example of chest and eye lenses, when scanning a patient, regions are produced in which a high noise level occurs in the imaging because of the low dose rate used.